In the production of pipes made of thermoplastic material intended for making fluid feed and/or drainage ducts (for example used in construction, in sewer systems, drinking water distribution networks, in drains) belling machines are used to form an end portion of the pipes into the characteristic “socket” shape, which is enlarged compared with the normal diameter of the pipe (the “connecting socket”) and is used to connect the pipes to each other one after another to form the duct. In general, an unformed end of a pipe is inserted in the socket end of the pipe which comes before or after it in the duct.
The belling machine, usually automatic, may be installed in an extrusion line and, in the line, receives the pipes/cut pieces of pipe to be processed.
Most belling machines make the socket with the thermoforming process. Belling machines are equipped with at least one oven for heating the end of the pipe to be formed and a forming apparatus which uses a suitable mould to form the heated end of the pipe into a socket. The socket formed on the mould is normally cooled in the forming apparatus. In the enlarged shape of common sockets, intended for connecting to another pipe, there is usually a seal groove for receiving an elastomeric seal which guarantees that the joint is sealed against leakage of fluids from the duct.
There is widespread use of the forming of the end of the pipe using a mandrel mould, also called a plug (in particular if using PVC-U pipes, or rigid polyvinyl chloride, not plasticised). The mandrel, or plug, reproduces the inner shape of the socket to be formed. To form the end of the pipe into a socket, the plug is inserted in the pipe at the end to be formed. The shape of the seal groove is usually obtained, once the mandrel has been inserted in the hot end of the pipe, by making expandable mechanical inserts come out of the surface of the mandrel. The inserts press on the inside of the pipe, forming the groove for the seal. Pipe forming is normally carried out and/or completed by acting on the end of the pipe fitted on the mandrel (when the mechanical inserts are expanded, if the seal groove is to be formed) with a fluidic action applied either by compressed air (or another suitable fluid) acting on the outside of the pipe, or by sucking with a vacuum from the inside of the mandrel. In general, fluidic action of air (or another suitable fluid) acting on the outside of the pipe is used. The wall of the pipe is formed against the mandrel. Once the socket has been formed and cooled, the mechanical inserts are retracted below the internal diameter of the socket, allowing extraction of the mandrel (or plug) from the socket.
The forming technique using a mechanical plug and pressurised fluid acting from the outside is widely used. Socket forming systems of this type are described, for example, in patent documents EP 0 684 124 and EP 0 516 595. In all of the systems with which a seal groove is made in the socket for the O-ring, after the socket has been formed, the seal must be inserted in the seal groove. It is inserted either manually or with the aid of tools. In particular in production lines for pipes intended for drains in buildings, machines are used which automatically insert the seal in the socket formed. In the production line these automatic seal inserters are usually located downstream of the automatic belling machine and they normally receive the pipe on which the socket has been formed directly from the belling machine.
There are special prior art automatic belling machines (often used in production lines for PVC-U pipes) which form an end socket with the seal already integrated in it. Such belling machines form the socket using a system commonly known in technical literature as the “Rieber system”.
In the Rieber system, a metal mandrel, on which the seal was previously placed in a precise position, is fitted at the heated end of the pipe. Consequently, the end of the pipe fits onto the metal mandrel and the seal. In practice, the mandrel and the seal together form the socket forming mould. Once the heated end of the pipe to be formed has been fitted on the assembly consisting of the mandrel and the seal positioned on the mandrel, the end of the pipe is definitively formed on the mandrel (and on the seal) by applying a vacuum (tending to create a vacuum inside the pipe fitted on the mandrel) and/or an overpressure on the outer surface of the pipe (for example with a pressurised fluid such as, in particular, compressed air). When cooling is complete, the seal remains locked in the socket, becoming an integral part of it.
The seals used in the Rieber system are different to the elastomeric seals normally intended for insertion in the seal groove of a socket after the socket has been formed and cooled. In particular, they are characterised by a structure which includes in the elastomeric material an annular part—made of metal or hard plastic—with specific seal strengthening and stiffening functions.
Therefore, the Rieber system allows a single machine to make a pipe whose end has a socket formed on it complete with a seal, the seal being locked in the pipe socket end. This avoids the need to manually insert seals in their grooves when installing the duct, and avoids having to equip the production line with the equipment necessary for inserting the seals in the respective grooves after the sockets have been formed. Therefore, the operating and logistical advantages of the Rieber system are obvious both during production and during duct installation.
However, one disadvantage of the Rieber system is the fact that any damage to the seal during socket forming, or incorrect positioning of the seal in the socket, results in the whole pipe having to be rejected, since the seal cannot be substituted in the pipe made using the Rieber system.
Compared with conventional belling systems (in which the seal is inserted in the socket after the socket has been formed and cooled), the Rieber system has the added disadvantage of requiring that a seal be loaded on the plug before the mandrel (or plug) is inserted in the heated end of the pipe. This is a disadvantage of the Rieber system compared with normal belling systems using a mechanical plug which form the socket with the seal groove but without inserting the seal in it. From the mechanical viewpoint, although the mandrel usable in the Rieber system is generally simpler than those which form the seal groove using expandable mechanical inserts (which are more complex and expensive), the Rieber belling system requires the presence, in the belling machine, of additional auxiliary devices, necessary for loading the seal on the mandrel (or plug). These additional devices are absent in conventional systems. From the point of view of production times, the step of loading the seal on the mandrel is carried out by means of a set of operations which require an execution time which affects the overall duration of the socket forming and cooling cycle.
Moreover, in the Rieber system the operations for loading the seal on the mandrel must guarantee a high level of reliability, so that the seal is actually and correctly positioned on the mandrel. The reliability of the loading process tends to be reduced with increases in the speed of the individual operations linked to the loading process. Greater speed in the loading operations causes a substantial increase in the complexity of the process control methods and an increase in the inertial forces (and the problems linked to them in terms of control and precision). It also accentuates impacts between parts which make contact with each other.
Significant examples of application of the Rieber system and of belling machines operating on the basis of that system are described in patent documents U.S. Pat. Nos. 4,030,872, 4,975,234, 4,204,823, 4,723,905.
In particular, document U.S. Pat. No. 4,030,872 describes a belling method in which a seal, having the features of those used in the Rieber system, is manually placed on the mandrel and held in position there during mandrel insertion in the end of the pipe by elements which can expand radially outwards from inside the mandrel. Said expandable elements are inserted in an annular cavity made on the inner surface of the seal where that inner surface makes contact with the surface of the mandrel and they hold the seal in position on the mandrel when the pipe is fitted onto it. Forming is completed by applying the vacuum inside the mandrel in the zone where the seal is positioned. Seal positioning on the mandrel before belling is not automated.
Document U.S. Pat. No. 4,975,234 describes a belling machine operating with a “Rieber” type system, in which a rigid cradle, open at the top, is connected to an actuator which moves it vertically from a lowered position, in which it does not interfere with the horizontal trajectory followed by the mandrel for its insertion in the end of the pipe, to a raised position in which it is positioned on said trajectory between the mandrel and the pipe. In the raised position, the cradle receives an annular seal, dropped onto it by a feed hopper into which the seal drops from a collection magazine. When the seal is in the cradle, the mandrel advances, enters the cradle and is inserted in the seal. The rigid cradle acts as an opposing element which holds the seal, allowing the mandrel to forcibly slide into the seal and diametrically expand it. While the mandrel begins entering the end of the pipe, the cradle is lowered. A contact flange coaxial with the mandrel advances, makes contact with the rear of the seal and puts it in position, holding it while the mandrel penetrates the pipe. Once the pipe has been fitted on the mandrel and the seal (and on part of the contact flange), the flange moves back and forming is completed. The mandrel is removed from the pipe, in which the seal remains, and the process can begin again.
In this system, the configuration of the seal collection cradle, which also acts as a rigid opposing element during loading on the mandrel, creates problems for centring the seal on the mandrel and makes free expansion of the seal difficult during the passage from the tapered leading portion of the mandrel to the cylindrical portion of the latter. These problems make it difficult to use this solution in rapid belling processes. Moreover, the position of the cradle and its movement make the solution incompatible with the processing of pipes having a medium—large diameter (greater than 630 mm), unless the belling machine (and in particular the forming mandrel) operates at heights above the ground that are much greater than the normal operating heights of pipe extrusion lines.
Document U.S. Pat. No. 4,204,823 describes a variant of the previous solution, in which the collection cradle, open at the top for receiving the seals dropped from the feed magazine, is made in two parts which, when the mandrel has loaded the seal using the cradle as a rigid opposing element, separate from each other in a horizontal direction, moving away from the axis along which the mandrel moves, by the action of a complex mechanism of levers and guides connected to the mandrel movement, thus allowing the mandrel to pass freely until it is inserted in the end of the pipe. The mandrel return to its initial position, after belling, causes the two parts of the cradle to close on each other again, so that the cycle can be started again. Although modifying the movement of the parts of the cradle, allowing the movement to be horizontal, the solution proposed is mechanically complex and does not solve the problem of centring the seal on the mandrel and the difficulty in seal free expansion during the passage from the tapered leading portion of the mandrel to its cylindrical portion. As already indicated, these problems make it difficult to use this solution in rapid belling processes.
Document U.S. Pat. No. 4,723,905 describes several variants of “Rieber” type seals and several devices for holding them in position on the mandrel while the mandrel, with the seal already in position, is forcibly inserted into the end of the pipe.
The problems encountered in all of the belling systems which can be traced back to the Rieber system which, to the Applicant's knowledge, have so far been made, make it practically impossible to adapt said system to conventional belling machines which have a high production capacity, due to the imprecise seal positioning (and, therefore, the high probability of error when the machine is made to operate rapidly) and/or due to the complications in the mechanisms (with the consequent difficulty integrating their operation into the overall operating cycle of the belling machine).
Of conventional belling machines (“non-Rieber” machines) where it might be worthwhile to integrate a “Rieber” type system, if it were rendered reliable and versatile enough for its operation to be able to integrate in the machine operating cycle without reducing its efficiency, the type of belling machine described in the above-mentioned patent document EP 0 684 124, in the name of the same Applicant as the present, should be included. That document describes the configuration of an automatic belling machine of significant interest for optimising the production process, as well as being suitable for processing short pipes (up to 0.5 m, to which the length of the socket is added). It typically comprises various stations to which the pipe is transported so that the various processing steps can be performed.
The first station receives the pipe cut along the extrusion line. In this station, a special positioner device moves the pipe longitudinally and detaches it from the pipe which comes after it, then stops the pipe in a precise position still aligned with the axis of extrusion.
From this station the pipe is then moved only transversally to other process stations and then, when the socket has been formed, the pipe is unloaded from the machine. The pipe is positioned in sequence in two heating stations and a forming station which are located alongside each other. When the pipe is positioned in the first heating station, the latter's oven, from the initial back position, advances towards the pipe until it encloses the end of the pipe to be heated. At the end of the first heating process, the oven moves back and the pipe released is moved transversally and positioned in front of the second heating station, in which the same movements as in the first heating station are performed.
When the heating process has ended, with the second oven in the back position, the pipe is positioned at the forming station, then clamped and locked by clamps, before the forming mandrel advances and penetrates the pipe. A forming chamber advances towards the pipe together with the mandrel. The chamber ends its stroke by resting against a contact structure applied to the pipe locking clamps. In this way, the chamber encloses the end of the pipe on which a socket is to be formed and creates a hermetic seal which allows chamber pressurisation with compressed air. The pressing action of the compressed air on the outer surface of the pipe presses and forms into a socket the wall of the end of the pipe against the plug. The plug precisely forms the inner shape of the socket. When socket forming and cooling are complete, the forming chamber moves back, the plug with the mechanical inserts retracted is extracted from the socket and the clamp opens. At this point the pipe is moved transversally so that it can be unloaded from the belling machine. The machine production rate is equal to the longest time for which the pipe remains in each station. Obviously, the system is optimised at the lowest production time when the times for which the pipe remains in the individual stations, including the time for movement from the previous station, are equal. The transversal dimension of the belling machine will be bigger the more operating stations there are (since these are positioned alongside each other). However, to make the machine faster, that is to say, to reduce the cycle time, it is convenient to split the heating and socket forming—cooling process over many stations, so that the time for which the pipe remains in each station is also split. For example, a time of 200 seconds for heating the end of the pipe so that it reaches the thermal state suitable for socket thermoforming, becomes a heating cycle time in a station equal to 100 seconds, if the heating is split over two ovens. Similarly, to reduce the cycle times, multi-belling solutions may be adopted: after the positioning station, the pipes may be gathered in groups each consisting of two or more pipes which are simultaneously moved and processed in the heating and socket forming—cooling stations. Owing to the size and complexity of the heating devices and forming moulds, this solution is only used for processing small diameter pipes, normally having diameters not greater than 160 mm.
The most complex and expensive part of the machine is the socket forming and cooling station. Socket forming and cooling stations which are split or configured for multi-belling would produce complex and heavy machines. Therefore, for the forming station the use of process and construction solutions which minimise process times for socket forming and cooling with reduced size is decisive.
This invention has for an aim to overcome the above-mentioned disadvantages, by providing a belling machine for pipes made of thermoplastic material, forming end sockets equipped with an integrated seal which makes seal loading on the forming mandrel precise, minimising the risk of errors.
Another aim of the invention is to provide a belling machine for pipes made of thermoplastic material, forming end sockets equipped with an integrated seal which allows high production rates with reduced margins of error.